Sodium cyanide is used in electroplating, treating metal surfaces, extracting and recovering metals from ores, and various chemical uses. It is produced by the neutralization of sodium hydroxide with hydrogen cyanide. Most often, producers use substantially pure anhydrous hydrogen cyanide to react with substantially pure sodium hydroxide. Hydrogen cyanide is produced commercially by various processes, for example, the Andrussow process, which catalytically reacts methane, ammonia and air. The synthesis product is a mixture of components, including the desired hydrogen cyanide as well as water, unreacted ammonia, hydrogen, nitrogen and oxides of carbon. Where substantially pure hydrogen cyanide is required, complicated and expensive rectification and isolation procedures are necessary to provide a satisfactory product.
Since there would be considerable savings in investment and operating cost if the procedures needed to purify hydrogen cyanide could be eliminated, there have been numerous attempts to use impure hydrogen cyanide gas to produce an aqueous cyanide solution for conversion to anhydrous sodium cyanide. However, when hydrogen cyanide synthesis gas is directly absorbed in sodium hydroxide, the aqueous solutions produced contain measurable quantities of impurities absorbed from the impure gases. One of the primary impurities in the aqueous solution is sodium carbonate formed by reaction of carbon dioxide with the sodium hydroxide neutralizing agent. Various processes have been used to remove the sodium carbonate before crystallization, or to decrease its formation by causing a different precipitant to form.
U.S. Pat. No. 4,847,062 teaches a continuous process for making sodium cyanide that employs a classifying crystallizer and an absorber to directly absorb hydrogen cyanide gas onto a cyanide solution without use of any agents to remove sodium carbonate. However, this process produces sodium cyanide having an approximate purity of 95% or less.
Typically, the sodium cyanide is formed into briquettes by dry compression methods and shipped to users who generally dissolve the sodium cyanide in water to make an aqueous solution to be used in their process. To be acceptable, crystals must have a high enough sodium cyanide concentration such that, when diluted, the weight percent sodium cyanide is high enough for the intended purpose.
Thus there is a need for an improved process for the production of sodium cyanide to obtain product of high purity levels from an impure hydrogen cyanide starting material. The present invention provides such a process.